1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance imaging method that can form a diffusion weighted image (to be called as DWI image hereinafter) and an apparent diffusion coefficient image (to be called as ADC image) obtained by diffusion weighted imaging, and an image processing apparatus that handles DWI images and ADC images.
2. Description of the Related Art
The magnetic resonance imaging is an imaging method in which nuclear spin of an object placed in a static magnetic field is magnetically exited with a high-frequency signal of its Larmor frequency, and an image is reconstructed from an MR signal generated along with the excitation.
The diffusion weighted imaging method is one of the imaging methods in the field of the magnetic resonance imaging, and DWI images that shows the diffusing manner (speed, direction, etc.) of water molecules in tissue. The DWI imaging is used to identify a tumor in, for example, the trunk of body and it has such an advantage of a high sensitivity and a high contrast as compared to those of the other imaging methods. This diffusion weighted imaging method has been made popular by the following publications, “Le Bihan D, MRM 19(2):221-227 (1991)”, “Takahara T, Radiat Med 22(4):275-282 (2004)” and “Tsunehiko Nishimura et al., diffusion•perfusion MRI (Ichibousennri (unlimited expansion of view):32, Medical View Co. 2006”. The last publication discloses DWI (Diffusion Weighted Imaging), especially “T2 shine-through”.
However, there is a tendency with the DWI imaging that a tissue with a large T2 value is extracted at a high signal even though it is not a tumor (which is the so-called “T2 shine-through”). (See “Burdette J H, Radiology 212:333-339(1999)”.) Under these circumstances, some special attention must be paid when visually analyzing a DWI image. On the other hand, when a DWI image is subjected to an MIP (maximum intensity projection) process, a tumor, in some case, is hidden behind some normal tissue of a high intensity signal. Further, although it is possible to a certain extent to distinguish a tumor from a normal tissue of a high intensity signal by measuring the ADC value with an ADC image formed from DWI images having different b factors from each other, such an observation based solely on the ADC value is not sufficient since the ADC value is an apparent diffusion coefficient, which is naturally not absolute one. Further, solely with the ADC image, it is difficult to make a judgment on the form of tissue itself. Furthermore, DWI and ADC are separate images, and therefore it is laborious to visually observe them for comparison and contrast.
As described above, it is conventionally difficult to read an image accurately with each individual image regardless of whether it is a DWI image or ADC image.
A more accurate reading of image can be achieved by comparing the DWI image and ADC image; however it is conventionally laborious to compare and contrast them since they are separate images.